A number of chemical products have hitherto been used to control vegetative root intrusion in pipeline systems. The most frequently used active ingredients in such products are copper sulfate, corrosive acids or bases, 2,6 dichlorobenzonitrile (hereafter “Dichlobenil”) and sodium methyldithiocarbamate (hereafter “Metam”). Each of these products, however, suffer from a number of disadvantages which render their use problematic.
For example, copper sulfate is not detoxified by wastewater treatment plants and, because it is a systemic herbicide, can damage above-ground vegetation. Corrosive acids and bases, such as sulfuric acid, hydrochloric acid, caustic soda, and sulfamic acid, use heat as the primary mechanism of root destruction, and as such are effective only at the point of application and do little to prevent regrowth. Dichlobenil acts upon growth points in root systems and therefore provides residual control by deterring regrowth, but has limited effectiveness, so that it is commonly formulated with the active agent Metam. Combinations of Metam and Dichlobenil were found to be so effective, they replaced other types of active ingredients in the industry. However, several problems associated with Metam were identified which created a motivation to find other suitable herbicides for root control. First, Metam is a carcinogen. Second, in concentrations typically used for root control, Metam may be toxic to microorganisms at biological wastewater treatment plants, where it may be particularly toxic to nitrifying bacteria. Third, Metam is a marine pollutant, and therefore is not amenable to storm drain applications unless costly precautions are taken to ensure against a release of Metam into fresh water sources such as streams, ponds and lakes.
U.S. Pat. No. 5,919,731 discloses the use of diquat as an effective agent against root growth in sewer pipes. Previously, diquat had been known as an herbicide which killed foliage by photosynthesis, and which was inactivated by organic materials. The mechanism by which diquat kills roots in sewers, where there is little or no photosynthesis occurring, and where there is an abundance of organic material, has not been determined. Although toxicity has been postulated to be associated with diquat's desiccant activity, this has not been confirmed. Toxicity issues associated with Metam are obviated by using diquat as a root control agent, because diquat's toxicity is limited due to its inactivation by organic matter.
Paraquat is chemically related to diquat, both being dipyridyl compounds. It is known in the art to share some, but not all, of the biological activities of diquat. For example, paraquat, like diquat, is an herbicide which acts by inhibiting photosynthesis and is inactivated by organic substances. However, the toxicology of paraquat and diquat are different. In animals, the primary target for paraquat toxicity is the lungs (Bismuth et al., 1982, J. Toxicol. Clin. Toxicol. 19:461-474), whereas diquat is severely toxic to the nervous system and does not produce significant pulmonary damage (Saeed et al., 2001, Postgrad. Med. 77(907):329-332; Lam et al., 1980, Toxicol. 18:111-123; Vanholder et al., 1981, Am. J. Med. 70:1267-1271).
Because the herbicidal mechanism shared by diquat and paraquat is not believed to operate in root control, it was not known, prior to the present invention, whether paraquat would be an effective root control agent. The fact that the primary toxic effects of diquat and paraquat in animals involve different organ systems contributed to this uncertainty, because it suggests that some tissues susceptible to damage by diquat are resistant to paraquat toxicity.